The World Health Organization issued sweeping new guidelines on Wednesday that could put millions more people on H.I.V. drugs than are now getting them. The recommendations could go a long way toward halting the epidemic, health officials say, but would cost untold billions of dollars not yet committed.

H.I.V. patients should be put on an antiretroviral therapy of three drugs immediately after diagnosis, the agency said, and everyone at risk of becoming infected should be offered protective doses of similar drugs.

Immediate treatment has become the standard of care in America and much of the developed world, but the agency’s new H.I.V. treatment and prevention guidelines increase by nine million the number of infected people who should get it worldwide.

The health agency did not estimate how many at-risk people would benefit from its new prevention guidelines, but Unaids, the United Nations AIDS fighting agency, made a back-of-the-envelope calculation that 10 million could be helped, including many women and girls in Africa not previously covered.

Dr. Mark Dybul, executive director of the Global Fund to Fight AIDS, Tuberculosis and Malaria, said the new recommendations were “critically important to moving us toward fast-track treatment and prevention goals.”

Numerous recent studies have shown that people taking so-called triple therapy every day not only live longer, but also have so little circulating virus that they are highly unlikely to infect others even through unprotected sex. Studies using Truvada, a two-drug combination taken preventively, have shown that those taking the drugs every day have near-total protection against infection.

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An Indian woman with H.I.V. shows her antiretroviral drugs at her home in New Delhi.CreditAndrew Caballero-Reynolds/Agence France-Presse — Getty Images

The recommendations underscored the difference in options available to patients in industrialized countries and those in the developing world, and public health advocates acknowledged that it was unclear where the money would come from to turn the new guidelines into reality. Donor contributions for AIDS have been essentially flat since 2009. Although the W.H.O. issues guidelines, each country sets its own policy. Inevitably, when treatment starts depends on how many citizens the country’s health budget can afford to treat. Fifteen million people are on treatment now, fewer than half of the 37 million people infected worldwide.

But advocates noted that the situation has appeared hopeless before; a generation ago, the idea of treating anyone in poor countries with $15,000 medications looked impossible. Now, with generic drugs and the generosity of wealthy nations, the number of people getting treatment in Africa, Asia and Latin America has been rising by about a million a year.

The new guidelines represent an acknowledgment that no vaccine is on the horizon, and that the long-touted “ABC” strategy — abstain, be faithful, use a condom — has not worked.

And yet there is hope the epidemic can be ended, or at least greatly shrunken, with tools now at hand.

“We can make the impossible possible,” said Dr. Deborah L. Birx, the United States global AIDS coordinator and head of the President’s Emergency Plan for AIDS Relief. “If we can demonstrate that this would break the back of the epidemic, I think people will step up.”

Clinical trials and experience in a few cities, like San Francisco and Vancouver, have shown that offering a mix of immediate treatment and prophylaxis with the newest H.I.V. drugs can cut down infection rates and the number of people spreading the virus.

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H.I.V.-positive patients, waiting last year at a clinic in the Thyolo District of Malawi, Africa, for antiretroviral consultations.CreditMarco Longari/Agence France-Presse — Getty Images

Unlike those drugs available 15 years ago, new ones have few harsh side effects and cost far less. A year of generic triple therapy made in India costs as little as $70. A generic version of Truvada, the drug used in PrEP, is available from India for about $70 a year.

The World Health Organization last issued new treatment guidelines in 2013, when it advocated immediate treatment for some groups, including children, pregnant women and people with tuberculosis. But for most others it recommended treatment only when their CD4 counts — a measure of immune system strength — fell below 500. Under those guidelines, about 28 million were eligible for treatment.

Although that meant starting before serious immune breakdown began — 500 is considered the lower limit of a healthy level — some advocates have argued for immediate treatment for all.

Failing to provide it meant losing patients and spreading the disease, they argued. The medical charity Doctors Without Borders said Wednesday that a third of people who got H.I.V. diagnoses in its clinics but were not put on drugs because they were not yet eligible never returned, presumably getting sicker and infecting others.

The new guidelines do not detail who should be offered preventive doses of drugs, known as PrEP, but “provisionally” define those at “substantial risk” as people for whom H.I.V. infection risk is “greater than 3 per 100 person — years.”

Many advocates have argued that groups not previously covered by treatment guidelines — particularly women and girls in those African countries where more than 1 percent of the adult population is infected — should be covered. The health organization’s previous guidelines recommended preventive doses for some gay men, transgender women, people with infected partners and others.

Risk guidelines must be determined individually, experts explained. Not all gay men need PrEP, for example; those in monogamous relationships with H.I.V.-negative partners do not, while those having unsafe sex with strangers do.

Poor women and girls in countries with high infection rates are considered at risk both from rape and from pressure to have sex without condoms with infected men in return for money, favors, grades, job promotions and so on.

Most African countries that depend on donors use a CD4 count of 500 — the old guideline — as the treatment starting point, as do India and China. Russia, Eastern Europe, Indonesia and some Southeast Asian countries use an even older W.H.O. guideline, waiting until patients fall below 350. Mexico, Brazil and a few other middle-income countries have already adopted test-and-treat protocols for everyone.

Although the W.H.O. endorsed PrEP, and it has been tested on people at risk in poor countries, it has been officially adopted almost nowhere in poor and middle-income countries. A Unaids spokeswoman estimated 20,000 Americans are on it, while fewer than 10,000 are elsewhere.

Estimating a price tag for the guidelines is difficult. The health organization did not do so in issuing them, other than acknowledging that countries would have to set priorities.

Almost $22 billion is currently spent on AIDS in poor and middle-income countries, half of it contributed by donors, according to Unaids. Even before the new guidelines, that was predicted to rise to $32 billion by 2020.

Dr. Dybul said Wednesday that he did not know how much the world would contribute.

“We can give a better answer in December,” he said, referring to plans for the board to meet in Tokyo for its next fund-raising appeal.

The United States is by far the largest donor. By law, it can contribute no more than a third of the global fund’s budget, but it has its own, larger program, Pepfar, which Dr. Birx oversees.

A version of this article appears in print on , on Page A8 of the New York edition with the headline: World Health Group Proposes Extending H.I.V. Therapy’s Reach. Order Reprints | Today’s Paper | Subscribe